The present embodiments relate to receive beamforming for medical diagnostic ultrasound. In particular, the receive beamformer includes an oversampling analog-to-digital converter for digital receive beamforming.
Oversampling converters (e.g., sigma-delta converters) digitize analog signals coarsely using a single or a few bits but at a sampling rate much higher than the Nyquist rate. Oversampling converters may be implemented more easily using modern VLSI technology compared to conventional Nyquist rate, multi-bit A/D converters. Using oversampling, the need for anti-aliasing filters and digital interpolation to reduce delay quantization error may be reduced.
In beamforming, dynamic receive focusing requires that channel signals be relatively delayed for coherent summation. The focusing delay is larger at shallower depths and smaller at deeper depths. The delay also varies with element position within an aperture. Implementing this dynamic delay change using the single-bit output stream from sigma-delta converters may be difficult. The data may be dynamically stretched where the amount of stretching varies with depth or time. FIG. 1A shows arrows indicating a location for stretching or extending the original single-bit sigma-delta output. One solution is to insert a zero where a stretch is needed as shown in FIG. 1B. Because zero insertion does not add “energy” to the data, the demodulated output is a good approximation of a stretched version of the original signal. However, three output states (1, 0, −1) and two output bits are required. Another approach is to repeat a sample (e.g., a 1 or −1) as shown in FIG. 1C. Repeating the previous sample does not require an extra bit, but adds “energy” to the bit stream that, after decoding, leads to noise in the reconstructed signal.
Various ways to address this problem have been described. U.S. Pat. No. 6,366,227 describes 3 approaches: 1) always add a +1 followed by a −1, which doubles the delay quantization error; 2) find symmetrical channels, add +1 to one channel and −1 to the other; or 3) keep track of all the +1 or −1's that have been added and subtract them later from the beam sum. U.S. Pat. No. 6,867,720 describes varying the feedback coefficient in a sigma-delta converter. For a sample that is to be repeated, this cuts feedback gain to ½ of normal gain. U.S. Pat. No. 6,895,123 describes separating the receive delay profile into a steering component and a focusing component. The steering component does not change with depth and poses no problem. The focusing component changes with depth but is symmetric about the beam origin. The focusing component is implemented in pairs by adding +1 in one signal and −1 in another signal that is symmetric with the first signal. U.S. Patent Application Publication Number 2004/0189499 describes an architecture of filtering the bit stream for each channel at the signal Nyquist rate, not the oversampling rate. The filtering is at a timing determined by dynamic receive focusing and occurs before summing the channels together. However, these techniques may be complex to implement.